Punch press workholder

ABSTRACT

An improved punch press workholder has a relatively high flexural rigidity in the X and Y directions and a relatively low flexural rigidity in the Z direction. The high flexural rigidity of the workholder in the X and Y directions enables it to hold a work sheet accurately in place during punching operations. The low flexural rigidity in the Z direction enables the work sheet to assume a proper position relative to a die under the influence of forces applied to the work sheet by a punch. The workholder includes a gripper having an upper or movable pivot arm which is pivotally connected with a lower or fixed pivot arm. A resiliently yieldable connection is integrally formed with the fixed pivot arm and connects the fixed pivot arm with a rigid base. The resiliently yieldable connection has a generally rectangular cross-sectional configuration with a small dimension measured in a direction parallel to the Z axis and a relatively large dimension measured in a direction parallel to the X axis so that the yieldable connection has a high flexural rigidity in the X direction and a low flexural rigidity in the Z direction.

United States Patent Taylor et al.

[451 Sept. 17, 1974 PUNCH PRESS WORKHOLDER [75] Inventors: Victor E.Taylor, Phoenixville;

Wallace R. Choate, Norristown; Stanley Banachowski, Collegeville; RobertW. Johnson, Philadelphia; Robert C. Faison, Ridley Park, all of Pa.

[73] Assignee: The Warner & Swasey Company, Cleveland, Ohio [22] Filed:June 6, 1973 [21] Appl. No.: 367,523

[52] US. Cl 83/146, 83/277, 83/412, 83/465, 83/566, 269/56 [51] Int. ClB26d 7/02 [58] Field of Search 83/146, 277, 566, 458, 83/409, 412, 465;269/56 [56] References Cited UNITED STATES PATENTS 3,230,806 l/l966Rovoldt 83/146 3,460,4I5 8/1969 Philipp 83/277 X 3,613,491 lO/l97lKahmann 83/l46 Primary ExaminerFrank T. Yost [57] ABSTRACT An improvedpunch press workholder has a relatively high flexural rigidity in the Xand Y directions and a relatively low flexural rigidity in the Zdirection. The high flexural rigidity of the workholder in the X and Ydirections enables it to hold a work sheet accurately in place duringpunching operations. The low flexural rigidity in the Z directionenables the work sheet to assume a proper position relative to a dieunder the influence of forces applied to the work sheet by a punch. Theworkholder includes a gripper having an.

upper or movable pivot arm which is pivotally connected with a lower orfixed pivot arm. A resiliently yieldable connection is integrally formedwith the fixed pivot arm and connects the fixed pivot arm with a rigidbase. The resiliently yieldable connection has a generally rectangularcross-sectional configuration with a small dimension measured in adirection parallel to the Z axis and a relatively large dimensionmeasured in a direction parallel to the X axis so that the yieldableconnection has a high flexural rigidity in the X direction and a lowflexural rigidity in the Z direction.

13 Claims, 7 Drawing Figures la: 60 i \\\\\\\v PMENTEBSEHTIQM sum 1 or 2I-TG. l.

PUNCH PRESS WORKHOLDER BACKGROUND OF THE INVENTION The present inventionrelates to a workholder and more specifically to a workholder which maybe utilized in a punch press and has a relatively low flexural rigidityin the vertical or Z direction to enable a work sheet to be raised to astripper of the press during a return stroke of a punch.

Turret-type punch presses commonly have workholders to hold a work sheetin a predetermined position relative to a die during punchingoperations. Known workholders, such as the workholders disclosed in U.S.Pat. Nos. 3,174,747 and 2,701,017, have a very rigid construction. Thisrigid construction is necessary in order to accurately position the worksheet relative to the X and Y axes of the punch press and retain thework sheet against movement during punching operations.

Disengagement of the work sheet from the punch of a press is facilitatedif the work sheet can move upwardly into engagement with a stripperunder the influence of forces applied to the work sheet by the punch.Once the work sheet has engaged the stripper, continued upward movementof the punch disengages it from the work sheet. The relative highstructural rigidity of known workholders in the vertical or Z directionhas prevented or substantially retarded upward movement of the worksheet under the influence of forces applied to it by the punch. Inaddition, the relatively high flexural rigidity of known workholders inthe vertical or Z direction prevents or retards downward movement of thework sheet toward a die during punching operations. This can result inmisalignment of the worksheet and die.

SUMMARY OF THE PRESENT INVENTION The present invention provides aworkholder which has a relatively high flexural rigidity in the X and Ydirections and a relatively low flexural rigidity in the vertical or Zdirection. The relatively low flexural rigidity of the workholder in the2 direction enables a work sheet to be moved downwardly into engagementwith a die during a working stroke of a punch. In addition the worksheet can move upwardly into engagement with a stripper under theinfluence of forces applied to the work sheet during a return stroke ofa punch. The relatively high flexural rigidijy of the workholder in theX and Y directions enables it to accurately position and hold a worksheet during punching operations.

The workholders relatively low flexural rigidity in the Z direction isobtained by providing a resiliently yieldable connection between agripper assembly and a rigid base of the workholder. The gripperassembly includes fixed and movable pivot arms having jaws whichclampingly engage a work sheet. The fixed pivot arm is connected withthe rigid base of the workholder by the resiliently yieldableconnection.

The resiliently yieldable connection is integrally formed with the fixedpivot arm and has a small crosssectional area. This cross-sectional areahas a very small height or Z axisdimension and a rather large base or Xaxis dimension. The small height or Z axis dimension results in arelatively low flexural rigidity in the vertical or Z direction.However, the large base of X axis dimension results in a relativelylarge flexural rigidity in the X direction.

To prevent the resiliently yieldable connection from being overstressedand plastically deformed, a stop arrangement is provided to limit theextent to which the connection can be flexed in the Z direction. Tofurther protect the resiliently yieldable connection, a guard isprovided to prevent the connection from being damaged by engagement withfalling workpieces and other foreign objects.

Accordingly, it is the object of this invention to provide a new andimproved workholder which is adapted to be utilized in a punch press andwhich has a relatively high flexural rigidity in the X and Y directionsto enable it to accurately position and hold a work sheet duringpunching operations and a relatively low flexural rigidity in thevertical or Z direction to enable the work sheet to be moved in avertical direction under the influence of forces applied to it by thepunch during a work and a return stroke of the punch.

Another object of this invention is to provide a new and improvedworkholder having a pair of arms which are pivotally connected at alocation intermediate their ends, a pair of work engaging jaws fixed toouter ends of the arms, an actuator for opening and closing the jaws,and a flexible connection which permits movement of the arms in the sameplane as in which they are pivoted by operation of the actuator.

Another object of this invention is to provide a new and improved punchpress for working on sheets of material and which includes a gripperassembly having clamp surfaces to grip a sheet of material and hold itagainst movement in a generally horizontal plane during a work stroke ofa punch and to enable at least a portion of the sheet of material tomove in a vertical or Z direction during either a work or return strokeof the punch and wherein the gripper assembly includes a resilientlyyieldable connection which interconnects the clamp surfaces and a baseportion of the gripper assembly to enable the clamp surfaces to movethrough a substantial distance in a vertical direction under theinfluence of forces applied to the sheet of material by the punch as thepunch is moved through either a work or return stroke.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects andfeatures of the present invention will become more apparent upon aconsideration of the following description taken in connection with theaccompanying drawings wherein:

FIG. 1 is partially broken away illustration of a turret punch press anddepicting the relationship between a punch assembly, a work sheetsupport table, and a pair of workholders constructed in accordance withthe present invention;

FIG. 2 is a plan view of one of the workholders of FIG. 1;

FIG. 3 is a sectional view, taken generally along the lines 33 of FIG.2, illustrating a gripper assembly of the workholder in an opencondition;

FIG. 4 is a partially broken away elevational view, taken generallyalong the line 4-4 of FIG. 2, illustrating the gripper assembly in aclosed condition;

FIG. 5 is an enlarge elevational view depicting the relationship betweena lower or fixed pivot arm, a resiliently yieldable connection, and arigid base portion of the gripper assembly;

FIG. 6 is a sectional view, taken generally along the line 66 of FIG. 5,illustrating the relatively small cross-sectional area of theresiliently yieldable connection; and

FIG. 7 is a sectional view, taken generally along the line 77 of FIG. 5,illustrating the relatively large cross-sectional area of the fixedpivot arm.

DESCRIPTION OF A SPECIFIC PREFERRED EMBODIMENT OF THE INVENTION A turretpunch press 10 is illustrated in FIG. 1 and includes a punch assembly 12for operating on a work sheet supported in a horizontal plane on a tableassembly 14. During punching operations, the work sheet is positionedand held against horizontal movement relative to the top of the tableassembly 14 by a pair of identical workholders l8 and 20. Theworkholders 18 and 20 have a relatively large flexural rigidity in the Xdirections, indicated by the horizontal arrow 24 in FIG. 1, and in the Ydirections, indicated by the horizontal arrow 26 in FIG. 1. Inaccordance with the present invention, the workholders l8 and 20 have arelatively low flexural rigidity in the Z directions, indicated by thevertical arrow 28. This relatively low flexural rigidity in the Zdirection enables the work sheet to be raised against a stripper 32under the influence of forces applied to the work sheet by a punch 34during a return stroke of the punch.

The punch assembly 12 is of a known construction and includes a guide 40which defines a cavity in which the punch 34 is disposed. During apunching operation, the punch 34 is moved downwardly relative to theguide and cooperates with a die 42 to form a hole in a work sheet in aknown manner. The relatively low flexural rigidity of the workholders l8and 20 in the Z di rection enables the work sheet to move downwardlyinto engagement with the die 42 during a punching stroke. Uponcompletion of the work or downward stroke, the punch 34 is retracted ormoved upwardly through a return stroke. As the punch 34 moves upwardly,frictional interference between the sides of the hole in the work sheetand the punch 34 cause the work sheet to be raised upwardly intoengagement with the stripper 32. Continued upward movement of the punch34 causes the stripper 32 to disengage the work sheet from the punch ina known manner. Although many different types of punch and stripperarrangements could be utilized, one suitable punch stripper arrangementis disclosed in US. Pat. No. 3,348,443.

The table assembly 14 is of known construction and supports the worksheet at die height during punching operations. The table assembly 14includes a pair of horizontal stationary support surfaces 48 and 50disposed on opposite sides of a disappearing table 52. The disappearingtable 52 includes a plurality of rollers 54 which are disposed in aspaced-apart parallel relationship and are interconnected by chain links60. The upper surfaces of the rollers 54 are tangent with the plane ofthe support surfaces 48 and 50 to continuously provide a movable supportfor the work sheet at die height. The construction of the table assembly14 is the same as is disclosed in U.S. Pat. No. 3,395,596 and will notbe further described herein to avoid prolixity of description.

A cross slide 64 is slidably mounted on a guide bar 66 which extendsparallel to the X axis. The workholders 18 and 20 are connected with thecross slide 64.

When the position of a work sheet gripped by the workholders 18 and 20is to be changed to the X direction, the cross slide 64 is moved alongthe guide bar 66 by a drive screw (not shown). When the position of thework sheet is to be changed in the Y direction, a Y-axis drive screw 70is operated to move a carriage 72 along guideways 73 in a directioneither toward or away from the punch assembly 12. Suitable drivearrangements for the cross slide 64 and carriage 72 are disclosed in US.Pat. No. 3,436,998 and will not be further described herein. However, itshould be noted that these drive arrangements hold the cross slide 64and carriage against movement in the X and Y directions during operationof the punch assembly 12 to enable the workholders l8 and 20 to hold awork sheet in a predetermined position relative to the punch assembly.

The workholder 18 includes a gripper assembly 74 which is operated by apneumatic actuator 76 (FIG. 2) to clampingly engage a work sheet 80 inthe manner shown in FIG. 4. The gripper assembly 74 includes a pair ofspaced-apart parallel lower or fixed pivot arms 84 and 86 (FIG. 2) towhich an upper or movable pivot arm 88 is pivotally connected by asuitable pin 90. Upon operation of the piston and cylinder type actuatorassembly 76 from the extended position of FIG. 3 to the retractedcondition of FIG. 4, the movable arm 88 is pivoted in a clockwisedirection (as viewed in FIG. 3) to clamp the work sheet 80 between upperand lower jaws 94 and 96.

The upper jaw 94 is formed on the outer end of the movable arm 88. Thelower jaw 96 is formed by a jaw plate 100 which extends between the arms84 and 86. Carbide inserts 102 may be provided on the upper and lowerjaws 94 and 96 to indent or dig into the work sheet 80 and provide afirm gripping action.

To enable the gripper assembly 74 to hold the work sheet 80 againstmovement in the X and Y directions during a punching operation, a rigidbase 106 of the gripper assembly 74 is fixedly connected with the crossslide 64 by a suitable connection 108 (see FIG. 3). A rigid spacer bar112 is fixedly connected with the base 106.

When the punch 34 (FIG. 1) is moved upwardly through a return stroke,the punch pulls the workpiece upwardly toward the stripper 32. Ofcourse, once the workpiece has engaged the stripper 32, continued upwardmovement causes the workpiece to be disengaged from the punch 34. Toenable the workpiece 80 to move upwardly under the influence of forcesapplied to it by the punch 34, the lower or fixed arms 84 and 86 of thegripper assembly 74 are connected with the base portion 106 of thegripper assembly by resiliently yieldable connection 116 (see FIGS. 4and 5). The connection 116 enables the workholder 18 to flex eitherupwardly or downwardly with the work sheet.

The resiliently yieldable connection 116 between the fixed pivot arm 86and rigid base 106 has a relatively low flexural rigidity in the Zdirection to enable the work sheet 80 to move upwardly into engagementwith the stripper 32 under the influence of forces applied to the worksheet 80 by the punch 34 as it moves through a return stroke. Thus, asthe punch 34 moves upwardly through a return stroke. it applies a forceto the work sheet 80 urging it upwardly toward the stripper 32. At leasta portion of this force is transmitted by the work sheet 80 to theclosed gripper assembly 74. Although the fixed pivot arms 84 and 86 arerelatively rigid, the

low flexural rigidity of the resiliently yieldable connections 116enables the pivot arms 84 and 86 to flex upwardly with the work sheet.Thus, the arm 86 is flexed from the position shown in solid lines inFIG. 5 to the position shown in dashed lines in FIG. 5. The yieldableconnections 116 also enable the arms 84 and 86 to flex in a verticaldirection during the punching stroke.

The relatively low flexural rigidity of the resiliently yieldableconnection 116 results from the fact that the connection has arelatively small cross-sectional area (see FIG. 6) in a central planewhich extends parallel to the X and Z axes and perpendicular to the Yaxis. Thus, the flexural rigidity (BI) is determined by the product ofYoungs modulus of elasticity (E) for the material of the yieldableconnection and the moment of inertia of the cross-sectional area withrespect to its neutral axis (I). Of course, Youngs modulus of elasticitywill be the same for the yieldable connection 116 whether flexuralrigidity about the X or Z axis is being determined. Therefore, thedifference between the flexural rigidity of the resiliently yieldableconnection 116 about the X axis and about the Z axis is determined bythe difference in the moment of inertia of the yieldable connectionabout these axes.

The moment of inertia of the resiliently yieldable connection 116 in theZ direction (I is calculated about a neutral axis 118 (FIG. 6) bymultiplying the base of the area times the cube of the height anddividing by 12. The size of the base in regard to the neutral axis 118is the length of the surface designated 120 in FIG. 6. The size of theheight in regard to the neutral axis 118 is the length of the surfacedesignated 122 in FIG. 6.

The base 120 is of a relatively large length while the height 122 is ofa relatively short length. Since the moment of inertia in the Zdirection is a function of the product of the base 120 times the cube ofthe height 122. the yieldable connection 116 has a relatively smallmoment of inertia about the neutral axis 118. Therefore, the yieldableconnection 116 has a relatively small flexural rigidity in the Zdirection.

The flexural rigidity of the yieldable connection 116 in the X directionis equal to Youngs modulus of elasticity (E) for the material of theconnection times the moment of inertia in the X direction. The moment ofinertia in the X direction (I is calculated about a neutral axisindicated at 124 in FIG. 6. When the moment of inertia about the neutralaxis 124 (It) is to be determined, the base is equal to the length ofthe surface 122 while the height is equal to the length of the surface120.

The base dimension 122 in regard to the neutral axis 124 is relativelysmall while the height dimension 120 in .regard to the neutral axis 124is relatively large. Therefore, the moment of inertia about the neutralaxis 124 (I is a function of the product of the relatively small basedimension 122 times the cube of the relatively large height dimension120. Thus, the moment of inertia about the neutral axis 124 (1;) is fargreater than the moment of inertia about the neutral axis 118 since theheight about the neutral axis 124 is far greater than the height aboutthe neutral axis 118.

The yieldable connection 116 is integrally formed with the fixed pivotarm 86. Although the crosssectional area of the arm 86 may vary alongits length, the cross-sectional area along line 77 of FIG. 5 is shown inFIG. 7. It should be noted that when the flexural rigidity in the Zdirection is to be determined, the moment of inertia is calculated aboutthe neutral'axis indicated at 130 in FIG. 7. The base dimension for thiscalculation will be determined by the length of the surface indicated at134 in FIG. 7. Since this surface is a continuation of the surface andis of substantially the same size, the base dimension for purposes ofcalculating moment inertia about the neutral axis of FIG. 7 is the sameas the base dimension for calculating the moment of inertia about theneutral axis 118 of FIG. 6.

The height for moment of inertia calculation about the axis 130 isdetermined by the length of the surface 138. As can be clearly seen fromcomparing FIGS. 6 and 7, the length of the surface 138 is far greaterthan the length of the surface 122. Therefore, the height utilized todetermine the moment of inertia about the neutral axis 130 is fargreater than the height utilized to determine the moment of inertiaabout the neutral axis 118. Accordingly, the resiliently yieldableconnection 116 will have a far smaller flexural rigidity in the Zdirection than the fixed pivot arm 86.

Although it is contemplated that the resiliently yieldable connection116 could have many different crosssectional configurations of differentsizes, in one specific preferred embodiment of this invention, theyieldable connection 116 had a surface 120 of a length of approximately0.75 inches and a surface 122 of a length of approximately 0.032 inches.With these dimensions, the moment of inertia about the neutral axis 118(I is equal to approximately 20.5 10"). The moment of inertia about theneutral axis 124 (I is far larger and equals approximately 11.25 (10Since Youngs modulus of elasticity for the material of the yieldableconnection 116 is constant, it can be seen that the flexural rigidity inthe Z direction will be equal to 20.5 (10' E. This is considerablysmaller than the flexural rigidity of the yieldable connection 116 inthe X direction which would be approximately I 1.25 (10) E.

In the specific preferred embodiment of the invention for whichdimensions are set forth above, the fixed pivot arm 86 has a much largercross-sectional area than the yieldable connection 116. Thus, thesurface 134 of the fixed pivot arm 86 had a length of approximately 0.75inches. However, the surface 138 had a length of approximately 0.25inches. Of course, the relatively large length of the surface 138compared to the length of the surface 122 results in the fixed pivot arm86 having a much greater flexural rigidity in both the X and Zdirections than the yieldable connection 116. The flexural rigidity ofthe yieldable connection 116 in the Y direction is extremely high. Thisis because forces in Y direction load the yieldable connection 116 ineither tension or compression.

Although specific dimensions have been set forth for one specificpreferred embodiment of the invention, it should be understood thatthese dimensions are provided to facilitate understanding of the presentinvention and it is not intended to limit the present invention to theseparticular dimensions. In addition, it should be understood thatalthough the yieldable connection 116 and fixed pivot arm 86 have beenillustrated in FIG. 6 and 7 as having rectangular cross-sectionalconfigurations, it is contemplated that they could have othercross-sectional configurations. It should be understood that althoughonly the yieldable connection 116 between the fixed pivot arm 86 andrigid base 106 has been illustrated in the drawings, a yieldableconnection of the same construction is provided between the fixed pivotarm 84 and the rigid base 106.

Although it is contemplated that the resiliently yieldable connections116 could be formed in many different ways, in one specific preferredembodiment of the invention, the fixed pivot arms 84 and 86, connections116, and base 106 were cast as a one-pice unit. As originally cast, theconnections 116 had the same cross sectional area and configuration asthe fixed pivot arms 84 and 86. The one-piece unit was then machined toform the connections 116 with a reduced cross sectional area. Thus, thebase 106, connections 116, and fixed pivot arms 84 and 86 are integrallyformed. However, it should be understood that the resiliently yieldableconnections 116 could have a construction other than the specificconstruction disclosed herein.

To prevent permanent or plastic deformation of the yieldable connection1 16, a stop arrangement 144 (see FIGS. 2 and 4) is provided to limitthe extent to which the connection can be deformed under the influenceof forces applied to the work sheet 80 by the punch 34. Thus, when theresiliently yieldable connection 116 is in its normal or undeformedcondition (illustrated in FIG. 4), a stop pin 146 is near the bottom ofa slot 148 formed in a guard member 150. When the fixed pivot arm 86 ismoved upwardly under the influence of forces applied to the work sheet80 by the punch 84, the pin 146 engages a stop or end surface 152 of theslot 148 to thereby limit upward movement of the fixed pivot arm 86.

The guard 150 has an upper section 164 (see FIG. 2) which extends overthe resiliently yieldable connection 116. The upper section 164 of theguard 150 protects the resiliently yieldable connection against beingdamaged by falling workpieces or other foreign objects. The inner end ofthe guard 150 is fixedly connected with the rigid base by suitableconnections 166 (FIG. 4).

Although the stop arrangement 144 has been shown separately from theguard 150, it is contemplated that the guard 150 could be provided witha leading end surface 170 (see FIG. 2) which is spaced from the pivotarm 86 by a predetermined distance to prevent excessive deflection ofthe fixed pivot arm 86. While only the stop arrangement 144 and guard150 associated with the fixed pivot arm 86 has been fully illustrated,it should be understood that a similar guard and stop arrangement isprovided in association with the fixed pivot arm 84.

In view of the foregoing description, it can be seen that the workholder18 has a relatively high flexural rigidity in the X and Y directions anda relatively low flexural rigidity in the Z direction. The relativelylow flexural rigidity of the workholder 18 in the vertical or Zdirection enables the work sheet 80 to be moved downwardly intoengagement with the die 42 during a work stroke of the punch 34. Inaddition, the relatively low flexural rigidity of the workholder 18 inthe Z direction enables the work sheet 80 to be moved upwardly intoengagement with a stripper 32 under the influence of forces applied tothe work sheet during a return stroke of the punch 34. The relativelyhigh flexural rigidity of the workholder 18 in the X and Y directionsenables it to accurately position and hold the work sheet duringpunching operations.

The relatively low flexural rigidity of the workholder 18 in the Zdirection results from the yieldable connections 116 between the fixedpivot arms 84 and 86 of the gripper assembly 74. The resilientlyyieldable connections 116 are integrally formed with the pivot arms 84and 86 and have relatively smallcross-sectional areas. Thecross-sectional configuration of the yieldable connections 116 is suchthat they have a very small height or Z axis dimension and a ratherlarge base or X axis dimension. The small height or Z axis dimensionresults in a relatively low flexural rigidity in the vertical or Zdirection. However, the large base or X axis dimension results in arelatively large flexural rigidity in the X direction.

We claim:

1. A workholder for a machine tool having a cross slide for positioninga workpiece, comprising in combination: first and second arm memberspivotally joined intermediate their ends, with one end of one of saidmembers being affixed to said cross slide; work engaging jaws affixed tothe other ends of said first and second arm members in opposingrelationship to each other; actuator means coacting with the first endsof said first and second arm members to pivotally open and close saidjaws; and flexure means intermediate the pivot point and the first endof one of said arm members to permit movement of said arm members in thesame plane as the pivoting movement.

2. Apparatus according to claim 1 wherein said flexure means is anintegral part of said one arm member.

3. Apparatus according to claim 2 wherein said flexure means is an areaof reduced cross-section.

4. A punch press for working on sheets of material, said punch pressincluding table means for supporting a sheet of material, said tablemeans including support surface means for supportingly engaging a majorside surface of the sheet of material, punch means for forming openingsin the sheet of material while it is disposed on said table means, diemeans for cooperating with said punch means, means for effectingmovement of said punch means through a work stroke in a first directiontransverse to said support surface means and for effecting movement ofsaid punch means through a return stroke in a second direction oppositeto said first direction, said punch means being effective to move atleast a portion of the sheet of material in one of said directionsduring at least a portion of one of said strokes, stripper means foreffecting disengagement of the sheet of material from said punch meansduring a return stroke by limiting movement of the sheet of material inthe second direction, and gripper means for holding the sheet ofmaterial against movement in a direction extending generally parallel tosaid support surface means during work and return strokes of said punchmeans and for enabling at least a portion of the sheet of material tomove in said one direction during at least one of said strokes of saidpunch means, said gripper means including clamp surface means forclampingly engaging the sheet of material during work and return strokesof said punch means, a base portion, resiliently yieldable connectionmeans for interconnecting said clamp surface means and said base portionand for enabling said clamp surface means to move relative to saidsupport surface means through a substantial distance in at least saidone of said directions under the influence of forces applied to thesheet of material by said punch means.

5. A punch press as set forth in claim 4 wherein said clamp surfacemeans includes first surface means for engaging one side of a sheet ofmaterial and second surface means for engaging another side of a sheetof material, said gripper means further including a first elongatedsupport portion connected with said first surface means and a secondelongated support portion connected with said second surface means, saidfirst and second elongated support portions having a substantiallygreater resistance to deflection in said one of said directions thansaid resiliently yieldable connection means.

6. A punch press as set forth in claim 5 wherein said resilientlyyieldable connection means is integrally formed with said firstelongated member.

7. A punch press as set forth in claim 4 wherein said gripper meansincludes an elongated arm extending between said resiliently yieldableconnection means and said clamp surface means, said arm having a firstcross sectional area in a plane extending perpendicular to thelongitudinal axis of said arm, said resiliently yieldable connectionmeans having a second cross sectional area which is substantiallysmaller than said first cross sectional area.

8. A punch press as set forth in claim 4 wherein said clamp surfacemeans includes a first surface means for engaging one side of the sheetof material and a second surface means for engaging an opposite side ofthe sheet of material, said gripper means further including a firstelongated arm having an inner end portion connected with saidresiliently yieldable connection means and an outer end portionconnected with said first surface means, a second elongated armconnected at an outer end portion with said second surface means, andconnection means for pivotally connecting said second arm with saidfirst arm at a location intermediate said inner and outer end portionsof said first arm.

9. A punch press as set forth in claim 8 wherein said inner end portionof said first arm is integrally formed with said resiliently yieldableconnection means and has a cross sectional area which is substantiallygreater than the cross sectional area of said yieldable connectionmeans.

10. A punch press as set forth in claim 4 further including stop meansfor limiting movement of said clamp surface means in the seconddirection under the influence of forces applied to the sheet of materialby said punch means as said punch means is moved through a returnstroke.

11. A punch press for working on sheets of material, said punch pressincluding table means for supporting a sheet of material, punch meansfor forming openings in the sheet of material while it is disposed onsaid table means, means for effecting movement of said punch meansthrough working and return strokes, and gripper means for holding thesheet of material during working and return strokes of said punch means,said gripper means including first and second arm members pivotallyjoined intermediate their ends, one end of one of said members beingconnected with said table, work engaging jaws affixed to the other endsof said first and second arm members in opposing relationship to eachother, actuator means coacting with the first ends of said first andsecond arm members to pivotally open and close said jaws, and flexuremeans intermediate the pivot point and the first end of one of said armmembers to permit movement of said arm members in the same plane as thepivoting movement.

12. Apparatus according to claim 11 wherein said flexure means is anintegral part of said one arm member.

13. Apparatus according to claim 12 wherein said flexure means is anarea of reduced cross-section.

1. A workholder for a machine tool having a cross slide for positioning a workpiece, comprising in combination: first and second arm members pivotally joined intermediate their ends, with one end of one of said members being affixed to said cross slide; work engaging jaws affixed to the other ends of said first and second arm members in opposing relationship to each other; actuator means coacting with the first ends of said first and second arm members to pivotally open and close said jaws; and flexure means intermediate the pivot point and the first end of one of said arm members to permit movement of said arm members in the same plane as the pivoting movement.
 2. Apparatus according to claim 1 wherein said flexure means is an integral part of said one arm member.
 3. Apparatus according to claim 2 wherein said flexure means is an area of reduced cross-section.
 4. A punch press for working on sheets of material, said punch press including table means for supporting a sheet of material, said table means including support surface means for supportingly engaging a major side surface of the sheet of material, punch means for forming openings in the sheet of material while it is disposed on said table means, die means for cooperating with said punch means, means for effecting movement of said punch means through a work stroke in a first direction transverse to said support surface means and for effecting movement of said punch means through a return stroke in a second direction opposite to said first direction, said punch means being effective to move at least a portion of the sheet of material in one of said directions during at least a portion of one of said strokes, stripper means for effecting disengagement of the sheet of material from said punch means during a return stroke by limiting movement of the sheet of material in the second direction, and gripper means for holding the sheet of material against movement in a direction extending generally parallel to said support surface means during work and return strokes of said punch means and for enabling at least a portion of the sheet of material to move in said one direction during at least one of said strokes of said punch means, said gripper means including clamp surface means for clampingly engaging the sheet of material during work and return strokes of said punch means, a base portion, resiliently yieldable connection means for interconnecting said clamp surface means and said base portion and for enabling said clamp surface means to move relative to said support surface means through a substantial distance in at least said one of said directions under the influence of forces applied to the sheet of material by said punch means.
 5. A punch press as set forth in claim 4 wherein said clamp surface means includes first surface means for engaging one side of a sheet of material and second surface means for engaging another side of a sheet of material, said gripper means further including a first elongated support portion connected with said first surface means and a second elongated support portion connected with said second surface means, said first and second elongated support portions having a substantially greater resistance to deflection in said one of said directions than said resiliently yieldable connection means.
 6. A punch press as set forth in claim 5 wherein said resiliently yieldable connection means is integrally formed with said first elongated member.
 7. A punch press as set forth in claim 4 wherein said gripper means includes an elongated arm extending between said resiliently yieldable connection means and said clamp surface means, said arm having a first cross sectional area in a plane extending perpendicular to the longitudinal axis of said arm, said resiliently yieldable connection means having a second cross sectional area which is substantially smaller than said first cross sectional area.
 8. A punch press as set forth in claim 4 wherein said clamp surface means includes a first surface means for engaging one side of the sheet of material and a second surface means for engaging an opposite side of the sheet of material, said gripper means further including a first elongated arm having an inner end portion connected with said resiliently yieldable connection means and an outer end portion connected with said first surface means, a second elongated arm connected at an outer end portion with said second surface means, and connection means for pivotally connecting said second arm with said first arm at a location intermediate said inner and outer end portions of said first arm.
 9. A punch press as set forth in claim 8 wherein said inner end portion of said first arm is integrally formed with said resiliently yieldable connection means and has a cross sectional area which is substantially greater than the cross sectional area of said yieldable connection means.
 10. A punch press as set forth in claim 4 further including stop means for limiting movement of said clamp surface means in the second direction under the influence of forces applied to the sheet of material by said punch means as said puNch means is moved through a return stroke.
 11. A punch press for working on sheets of material, said punch press including table means for supporting a sheet of material, punch means for forming openings in the sheet of material while it is disposed on said table means, means for effecting movement of said punch means through working and return strokes, and gripper means for holding the sheet of material during working and return strokes of said punch means, said gripper means including first and second arm members pivotally joined intermediate their ends, one end of one of said members being connected with said table, work engaging jaws affixed to the other ends of said first and second arm members in opposing relationship to each other, actuator means coacting with the first ends of said first and second arm members to pivotally open and close said jaws, and flexure means intermediate the pivot point and the first end of one of said arm members to permit movement of said arm members in the same plane as the pivoting movement.
 12. Apparatus according to claim 11 wherein said flexure means is an integral part of said one arm member.
 13. Apparatus according to claim 12 wherein said flexure means is an area of reduced cross-section. 